Downhole wells for the recovery of hydrocarbon fluids, such as oil and/or natural gas from the earth or for the capture of heat energy powered by aqueous fluid present in the earth are operating at higher and higher temperatures, depending where the well is drilled and how deep. With deep drilling, temperatures of at least 280° C. are not uncommon at or near the bottom of the well, i.e. proximate to the bottom of the well.
Communications cable is inserted into these downhole wells for passing signals between a control unit on the earth's surface and a downhole tool, such as a logging sensor, or to electrically power downhole operations, such as drilling. The cables include a polymeric component, e.g. either as electrical insulation or in the case of optical fiber cable as protective material surrounding the optical fiber, i.e. jacket and/or filler material positioned between the optical fiber and jacket. The downhole tool itself can include a polymeric component as sealing material to prevent intrusion of downhole fluids into the tool, insulation for electrical conductor(s) or protective material surrounding the optical fiber.
The polymeric component is the weak link in the cable insofar as ability to withstand the higher and higher temperatures encountered proximate to the bottom of the well as wells are bored deeper and deeper into the earth. U.S. Pat. No. 5,894,104 discloses a slick line cable having a logging sonde at its lower end, the cable including polymer insulations such as PFA, FEP, and TEFZEL®, and the sonde including seals of elastomer or PEEK (polyether ether ketone). PFA is tetrafluoroethylene (TFE)/perfluoro(alkyl vinyl ether) copolymer, FEP is TFE/hexafluoropropylene copolymer, and TEFZEL® is TFE/ethylene copolymer (ETFE). U.S. Pat. No. 7,235,743 discloses in FIG. 5 a wellbore cable that includes a plurality of polymer components, electrical insulation 506 surrounding a twisted strand of electrical conductors, compression-resistant filler rods 508 that may be surrounded by compression resistant polymer, armor wires 516 and 518 forming the exterior of the cable, which have a polymer coating, a jacket 514 surrounding the assemblage of insulated conductors and filler rods, and filler material 510 filling the space between insulated wires, filler rods, and jacket. PFA, FEP, and ETFE are among the polymers disclosed as being useful for many of these applications in the cable. These fluoropolymers as commonly available have the following temperature characteristics*:
Melting Temp. (° C.)Continuous Use Temp. (° C.)PFA302-310260FEP245-265200ETFE250-280150* pp. 6 and 125-128 and 133-134 of S. Ebnesajjad, Fluoroplastics, Vol. 2 Melt Processible Fluoropolymers, The Definitive User's Guide and Databook, published by Plastics Design Library (2003). The PFA melting temperature is for the PFA commercialized in 1972 (tetrafluoroethylene/perfluoro (propyl vinyl ether).The melting temperature is the temperature corresponding to the position of the DSC endothermic peak resulting from the phase change of the polymer from the solid to the liquid (molten) state. The temperature that can be withstood by the polymer is far less than the melting temperature, however, as indicated by the much lower continuous use (service) temperatures. The continuous use temperatures reported above are understood to be the highest temperature at which the polymer can be used over a period of time of 6 months, during which time the tensile property falls to 50% of its original value. This temperature is determined by tensile property testing of no-load heat aged test samples of the polymer for 6 months. The test samples are removed from the heat-aging oven and are subjected to tensile property testing at ambient temperature (15-25° C.).
The reduction in tensile property with increased heat aging time denotes a deterioration of the integrity of the fluoropolymer. When the fluoropolymer is tensile-tested at the temperature of heat aging, however, the reduction in tensile property is immediate, i.e. aging is not required. For example, the tensile strength of ETFE of at least about 6000 psi (41.4 MPa) at ambient temperature falls to about 2000 psi (13.8 MPa) at 150° C. tensile testing. The concern about high temperature tensile property reduction manifests itself in the industry standard for annealing temperature for the highest melting melt processible fluoropolymer, PFA, listed in the table above. Molded articles of PFA are heated to 250-260° C. in order to relieve internal stresses to improve dimensional stability of the molded article. A typical heating time of 10 min for each 1 mm of thickness results in a heating time of about 4 hr for a thick-walled PFA molded article, i.e. 25.4 mm thick. The heating temperature is kept far below the melting temperature of the PFA to avoid collapse of the article.
PFA, having the highest continuous service temperature makes it the choice for the components of articles used in downhole wells proximate to the bottom of the well. From the experience with annealing PFA articles without causing them to become dimensionally unstable under their own weight, the continuous use temperature has become understood as being the upper limit on service temperature.
The question is whether any fluoropolymer can be used continuously at temperatures above 260° C. Polytetrafluoroethylene (PTFE) could conceivably be a candidate, because of the fact that PTFE does not flow in the molten state, due to its extremely high molecular weight. PTFE, however, has the disadvantage that it cannot be fabricated into articles by melt extrusion. This lack of melt processability is a practical barrier to the use of PTFE as the polymer component(s) of communications cable in downhole wells. Cable insulation, jacket, and protective material require melt extrusion to form their long lengths necessary in the manufacture of the long lengths of cable needed in downhole wells.